(1) Field of the Invention
This invention relates to a bus-method communication network system comprising a plurality of stations connected by a bus-method transmission path so as to commonly use the transmission path for mutual communication of the stations.
(2) Description of the Prior Art
A local area network communication system has been recently developed in order to connect a plurality of stations which are distributed in a relatively narrow area. A bus-method communication network system is a typical one of the local area network system.
The bus-method communication network system comprises a bus-method transmission path and the stations which are assigned with station numbers different from one another. The stations are commonly connected to the transmission path so as to commonly use the transmission path for mutual communication of the stations.
In the bus-method communication network system, contention occurs for seizing a transmission right or priority as is well known in the art. In order to resolve the contention, various methods have been already known in the art for accessing the transmission path, or seizing an exclusive transmission right.
A known access method is a carrier sense multiple access with collision detection (CSMA/CD) method. In the CSMA/CD method, each station always performs carrier detection on the transmission path. One station transmits a signal after confirming that none of other stations transmits any signal. On detection of collision, the station stops transmission of the signal and tries to retransmit the signal after the lapse of a random time period. However, the CSMA/CD method is disadvantageous in that processing is complicated for seizing the transmission right.
A polling protocol is another access method. In the polling protocol, a master controller is connected to the transmission path. The master controller transmits a polling sequence to one of the stations in order to poll or interrogate about absence or presence of a transmission request. The polled station transmits a data signal when the transmission request is present, while the polled station transmits a negative acknowledge in absence of the transmission request. Accordingly, the polling protocol is disadvantageous in that the bus-method communication network system is expensive because the master controller is necessary for polling the stations. In addition, it is impossible to effectively use the transmission path for increased number of stations. This is because the controller spends a lot of time for polling in comparison with actual communication.
A token passing method is still another access method. In the token passing method, a token is transferred along a predetermined logical ring of the stations one after another. A specific station in the logical ring seizes the exclusive transmission right by capture of the token. Then, the specific station sends out communication data to the transmission path if there are any, and sends out the token after the data are completely transmitted. In absence of data which should be transmitted, the specific station immediately transmits the token. In the token passing method, extinction and duplication of the token should be observed. Therefore, one or more supervising stations must be provided for monitoring the token and recovering the token from such faults, or one or more stations must be arranged to have such a supervising function. In addition, the logical ring must be dynamically determined for permitting another station to join to, or retire from, the logical ring, and the resultant logical ring must be maintained without fault. Accordingly, the token passing method is disadvantageous in that the bus-method communication network system is very expensive because the supervising is a very complex and difficult processing.